Membrane Transport and Facilitated Diffusion

Substances with easy passage across the plasma membrane

Small nonpolar, hydrophobic molecules

• Examples:

- CO₂, O₂, N₂

Substances with difficult passage across the plasma membrane

Hydrophilic, polar molecules, large molecules, ions

• Examples

- Sugars, water

Two main types of transport across membrane:

Passive and active transport

Passive Transport

transport of a molecule that does not require energy from the cell because a solute is moving with its concentration or electrochemical gradient

• Involved in the import of materials and export of waste

Types of Passive Transport

• Diffusion

• Osmosis

• Facilitated Diffusion

Diffusion

spontaneous process resulting from the constant motion of molecules

• substances move from a high to low concentration

• Move DOWN the concentration gradient

• Molecules diffuse directly across membrane

→ Different rates of diffusion for different molecules

Even with diffusion, the membrane is still…

selectively permeable

Osmosis

the diffusion of water down its concentration gradient across a selectively permeable membrane

• can also be thought of as the diffusion of water from areas of low solute concentration to areas of high solute concentration

Facilitated diffusion

diffusion of molecules through the membrane via transport proteins

What substances does facilitated diffusion increase rates of diffusion for?

• Small ions

• Water

• Carbohydrates

Two categories of transport proteins

1) Channel Proteins

2) Carrier Proteins

• Each transport protein is specific for substances it can facilitate movement for

Why is facilitated diffusion passive transport and not active transport?

Because the substances the proteins are helping to move are moving DOWN their concentration gradient and NO energy is required

Channel Proteins

provide a channel for molecules and ions to pass

• channel is hydrophilic

• only allow passage when there is a stimulus

• Aquaporins fall under this category

Aquaporins

specific channel protein for water

• if it ceases to function, water molecules will still be able to move across the cell membrane but at a slower rate

Carrier proteins

undergo conformational changes for substances to pass

Active transport

transport of molecule that requires energy because it moves a solute against its concentration gradient

• Pumps

• Cotransport

• Exocytosis

• Endocytosis

Active transport requires…

energy (ATP)

ATP

Adenosine Triphosphate (ATP)

• Energy source used by cells

• Can transfer the terminal phosphate group to the transport protein, which changes the shape of the transport protein to better move the substance

Conformational change

change in shape

Pumps

maintain membrane potential

Membrane potential

unequal concentrations of ions across the membrane results in an electrical charge (electrochemical gradient)

• The cytoplasm is relatively negative in comparison to extracellular fluid

• Energy is stored in electrochemical gradients

Electrogenic pumps

proteins that generate voltage across membranes, which can be used later as an energy source for cellular processes

• Example of this is the sodium potassium pump

Sodium-potassium pump

Animal cells will regulate their relative concentrations of Na+ and K+

• 3 Na+ get pumped out of the cell

• 2 K+ get pumped into the cell

→ results in a +1 net charge to the extracellular fluid

Proton pump

integral membrane protein that builds up a proton gradient across the membrane

• Used by plants, fungi, and bacteria

• Pumps H+ out of the cell

Cotransport

the coupling of favorable movement of one substance with an unfavorable movement of another substance

• Uses the energy stored in electrochemical gradients (generated by pumps) to move substances against their concentration gradient

• The energy stored in these electrochemical gradients is released when the substance start to move back down (with) their concentration gradient

• Plants use cotransport for sugars and amino acids

• Ex: Sucrose-H+ cotransporter

→ sucrose can travel into the plant against its concentration gradient only if it is coupled with H+ that is diffusion down its electrochemical gradient

Favorable movement/unfavorable movement

downhill diffusion/uphill transport

Transport of large molecules is done through

exocytosis and endocytosis

Exocytosis

the secretion of molecules via vesicles that fuse to the plasma membrane

• vesicle can fuse to the membrane by forming a bilayer

• once fused, the contents of the vesicle are released to the extracellular fluid

• Ex: nerve cells releasing neurotransmitters

Endocytosis

the uptake of molecules from vesicles fused from the plasma membrane (think: opposite of exocytosis)

Phagocytosis

when a cell engulfs particles to be later digested by lysosomes

• Cell surrounds particle with pseudopodia

• Packages particles into a food vacuole which fuses with the lysosome to be digested

Pinocytosis

nonspecific uptake of extracellular fluid containing dissolved molecules

• cell takes in dissolved molecules in protein coated vesicle

• protein coat helps to mediate the transport of molecules

Receptor mediated endocytosis

specific uptake of molecules via solute binding to receptors on the plasma membrane

• Allows the cell to take up large quantities of a specific substance

• When solutes bind to the receptors they cluster in a coated vesicle to be taken into the cell